"Mr. BATFE agent, I traded that lower to another individual years ago and had no reason to believe he was a prohibited person. What he did with it past then was his own business, and I have no knowledge of what he might have done.
Am I being detained? Am I free to go?"
But you'll still have an AR lower, just not the one that is being investigated.
So, if they were to search your home or whatever if the lower was there anyways then they would find one(in a confiscation scenario).
If you're talking about investigating the particular lower based on some other circumstance then theoretically that shouldn't have happened while in your possession anyways. It just seems symbolic to me.